Low-temperature fabrication of glass optical components

ABSTRACT

In one aspect, a method is provided for molding from glass complex optical components such as lenses, microlens, arrays of microlenses, and gratings or surface-relief diffusers having fine or hyperfine microstructures suitable for optical or electro-optical applications. In another aspect, mold masters or patterns, which define the profile of the optical components, made on metal alloys, particularly titanium or nickel alloys, or refractory compositions, with or without a non-reactive coating are provided. Given that molding optical components from oxide glasses has numerous drawbacks, it has been discovered in accordance with the invention that non-oxide glasses substantially eliminates these drawbacks. The non-oxide glasses, such as chalcogenide, chalcohalide, and halide glasses, may be used in the mold either in bulk, planar, or power forms. In the mold, the glass is heated to about 10-110° C., preferably about 50° C., above its transition temperature (Tg), at which temperature the glass has a viscosity that permits it to flow and conform exactly to the pattern of the mold.

FIELD OF THE INVENTION

The present invention pertains to optical components and theirmanufacture in glass. In particular, the invention relates to methodsand apparatus for molding or embossing non-oxide glasses (e.g.,chalcogenide glasses) with low glass-transition temperatures (T_(g))into devices of varied geometry having complex or fine surface features.

BACKGROUND

Optical elements have had various uses in many diverse technologies,including sensors, image projectors, displays (e.g., liquid crystaldisplay (LCDs), plasma display, and electro-luminescence display), aswell as opto-electronic devices for telecommunications. As thetelecommunications industry develops, the need to develop precisionoptical elements that incorporate microstructures increases. Intelecommunication devices, optical elements may be used, for instance,in fiber and laser couplers, optical switches, or as diffractiongratings for WDM applications, and densely packed microlens arrays(MLAs) or networks for wavelength management modules or collimatorapplications. Precision optical elements require highly polishedsurfaces or exacting surface figures and qualities. The surfaces demandfabrication in proper geometric relationship to each other; and, wherethe elements are to be used in transmission applications, they will beprepared from a material of controlled, uniform, and isotropicrefractive index.

Numerous methods and materials may be used to fabricate complex,precision optical elements. Because a great majority of conventionalmachining processes for the manufacture of optical components areunsuited for producing very small features, components having surfacefeatures or dimensions of 500 microns or smaller typically can befabricated only through a few methods of limited applicability.Fabrication of microstructured surfaces using polymers have leveragedoff of processes developed by the semiconductor industry for makingintegrated circuits. Using photolithography and ion etching techniques,some have created submillimeter surface features. These methods,however, are not conducive to large-scale manufacturing. The processtime needed to etch a microstructure is proportionally dependent on therequired total depth of the microstructure. Moreover, the methods arenot only expensive, but can produce only a limited range of featuretypes. Also, etching processes can create rough surfaces. A smoothconcave or convex profile or true prismatic profiles cannot be readilyachieved using either of the two aforementioned techniques.

Molding or hot embossing of plastics or glass materials, on the otherhand, can form submillimeter-sized features. Plastics can conform tomolds and reproduce faithfully intricate designs or finemicrostructures. Unfortunately for many telecommunication applications,plastic materials are not ideal since they suffer from severalshortcomings. Plastic materials are often not sufficiently robust towithstand, over time, environmental degradation. First, they exhibitlarge coefficients of thermal expansion, and limited mechanicalproperties. Plastic optical devices often cannot long withstand humidityor high temperatures. Both the volume and refractive indices of plasticscan vary substantially with changes in temperature, thereby limiting thetemperature range over which they may be useful. Plastics cannottransmit high-power light, due to internal heating of the material.Thus, well before a plastic component actually melts, its surfacefeatures will degrade and its index of refraction may change. Eitherchange is unacceptable in an optical context. Furthermore, sinceplastics for optical applications are available in a limited range ofdispersion and refractive index, plastics can provide only a restrictedtransmission range. Hence, their usefulness even within the restrictedbandwidth is limited by the tendency to accumulate internal stresses, acondition that results in distortion of transmitted light during use. Inaddition, many plastics can scratch easily and are prone to yellowing ordeveloping haze and birefringence. Application of abrasive-resistant andanti-reflective coatings, unfortunately, still has not fully solvedthese flaws. Finally, many chemical and environmental agents degradeplastics, which makes them difficult to clean effectively.

In comparison, glass possesses properties that make it a better class ofoptical material over plastics. Glass normally does not suffer from thematerial shortcomings of plastics, and it can better withstanddetrimental environmental or operational conditions. Hence, glass is amore preferred material. Glass optical components represent a differentclass of devices than those made from plastics and the molding processesused are more stringent.

Precision optical elements of glass are customarily produced by one oftwo complex, multi-step processes. In the first, a glass batch is meltedat high temperatures and the melt is formed into a glass body or gobhaving a controlled and homogeneous refractive index. Thereafter, theglass body may be reformed using repressing techniques to yield a shapeapproximating the desired final article. The surface quality and finishof the body at this stage of production, however, are not adequate forimage forming optics. The rough article is fine annealed to develop theproper refractive index and the surface features improved byconventional grinding and polishing practices. In the second method, theglass melt is formed into a bulk body, which is immediately fineannealed, cut and ground into articles of the desired configuration.

Both of these methods have their limitations. On one hand, grinding andpolishing are restricted to producing relatively simple shapes, such,such as flats, spheres, and parabolas. Other shapes and general asphericsurfaces are difficult to grind and complicated to polish. On anotherhand, conventional techniques for hot pressing of glass do not providethe exacting surface features and qualities, which are required forclear image forming or transmission applications. The presence of chillwrinkles in the surface and surface figure deviations constitute chronicafflictions.

The molding of glass traditionally has presented a number of otherproblems. Generally, to mold glass one must use high temperatures,typically greater than about 700° C. or 800° C., so as to make the glassconform or flow into a requisite profile as defined by a mold. First, atsuch relatively high temperatures, glass becomes highly chemicallyreactive. Due to this reactivity of glass, highly refractory molds withinert contact surfaces are required. Some materials used to fabricatemolds include silicon carbide, silicon nitride or other ceramicmaterials, or intermetallic materials, such as iron aluminides, or hardmaterials, such as tungsten. In many cases, however, such materials donot present sufficient surface smoothness or optical quality for makingsatisfactory optical surface finishes. Precision optical elementsrequire highly polished surfaces of exacting microstructure and quality.Metal molds can deform and re-crystallize at high temperatures, whichcan adversely affect the surface and optical qualities of the articlebeing molded. This means additional costs to repair and maintain themolds and higher defects in the product. Second, also due to thereactivity of the glass at high temperatures, often the molding need tobe done in an inert atmosphere, which complicates the process. Third,the potential for air or gas bubbles to be entrapped in the moldedarticles is another drawback of high-temperature molding. If capturedwithin the glass, gas bubbles tend to degrade the optical properties ofthe article. The bubbles distort images and generally disrupt opticaltransmission. Fourth, even at high temperatures, hot-glass moldingcannot create efficiently on the surface intricate, high-frequency,submillimeter microstructures, such as those required for diffractiongratings.

In the past, workers in the field of molding technology have endeavoredto develop several techniques for the manufacture of optical elements.These techniques, however, have yet to satisfactorily overcome thedeficiencies of glass molding. Hence, a new method or an improvement ofexisting technology is needed to for the manufacture of precisionoptical elements with deep or fine microstructures, such for diffractiongratings or microlenses. The method should be cost-efficient, expedientand enable high-volume, mass production of fine-figured microstructuresin multiple, identical glass optical elements. The present invention cansatisfy these needs.

SUMMARY OF THE INVENTION

The present invention pertains, in part, to a cost-effective method ofmaking a precision optical element with fine optical microstructures bymeans of molding or embossing. It has been discovered in accordance withthe present invention that the drawbacks associated with glass moldingcan be substantially eliminated through the use of non-oxide basedglasses as the material to be molded. Suitable glass compositionsinclude chalcogenide, chalco-halide, and halide glasses, which typicallyall have low glass transition temperatures (T_(g)). An example of ahalide glass may be a fluoro-zirconate glass (e.g., ZBLAN). Of the threekinds of glass, a sulfide glass is preferred, or more particularly, agermanium-arsenic-sulfide glass. The advantages of a non-oxide orchalcogenide glass includes high refractive index, lower moldingtemperature, excellent thermal stability and good environmentaldurability. The high refractive indices of these glasses areparticularly beneficial, since they reduce the extent of sag requiredfor making a lens of a given focal length. The low molding temperaturesof these glasses are attractive because they obviate the need forexpensive molds or masters, such as chemically vapor deposited siliconcarbide or silicon nitrides, required for molding higher-temperatureoxide glasses.

In brief, the method of manufacture comprises several steps. First,provide a non-oxide glass with a Tg of up to about 550° C. Second,provide a mold having at least a first portion and a second portion. Atleast an active, molding surface is formed of a material selected from:titanium alloys; nickel alloys; silicon carbide; silicon nitride; orrefractory ceramic composite of silicon carbide and silicon nitride, ora refractory metal such as tungsten and its alloys. The mold componentshave an active surface that has an optical finish and can be used withor without a protective coating a protective coating. Place the glass inthe mold. Then, heat the glass, the mold, or both to an operationaltemperature from about 10° C. to about 110° C. above the T_(g). Pressthe mold when the viscosity of the glass reaches about 10⁶-10¹² poise.At room temperature, the glass may take the form of granular, planar,bulk-solid items (e.g., respectively, a powder frit, a wafer or planar(disk) body, a bulk-solid ingot or monolith of any practicalthree-dimensional shape), or a combination thereof. When the glass is inthe form of a wafer or powder, the method further comprising insertingblocks into the mold. When blocks are used, the method may furthercomprise applying or placing a layer of material, which is non-reactivewith the glass at the operational temperature, on a surface of theblocks that is in contact with the glass material. This release coating,such as a boron nitride, may be spray coated or sputtered on thesurfaces of the mold defining the profile or the master defining thepattern. The method further comprises the steps of hardening the glassin the mold either through natural or forced cooling, then removing theglass. Further processing of the resultant embossed or molded glassarticle may be included, such as fine annealing or polishing. Thepresent method permits the glass to be molded in an atmosphere thatcontains oxygen, such as ambient air, as well as enclosed inconventional inert gas atmospheres.

Once heated to the operational temperature, under pressure, the glasssags into the mold to conform to a master design, whereby thesurface-relief structure of the master is transferred into the glass.When the starting materials are granular in form, such as glass frit,the molding process can sinter the individual glass particles into asolid article without trapping air pockets or other occlusions, whichmay mar the final product. Unlike certain methods indicated before suchas, grinding, polishing, reactive ion etching, which are based onprecise material removal processes, in the present fabrication process,fabrication times are not dependent on, nor directly determined by thedepth of the microstructure.

In another aspect, the present invention relates to a mold assemblycomprising a first or upper component and a second or lower component.The mold may be made using a variety of materials, which may include forexample silicon carbide; silicon nitride; a refractory ceramic orcomposite of the two or more metals, alloys, ceramics and glass. Apreferred material is a titanium alloy of a nominal composition, interms of weight percent, consisting essentially of about: 80-98% Ti(titanium); 1-10% Al (aluminum); and 1-10% V (vanadium). Titanium alloysof such compositions have been used in military aircraft compressors andbio-implants but not employed as mold materials for glass moldings.Surface treatment of Ti-6Al-4V alloys, such as nitriding, can improvesurface wear properties of the material.

Additional features and advantages of the present molding/embossing ofglasses will be explained in the following detailed description. It isunderstood that both the foregoing general description and the followingdetailed description and examples are merely representative of theinvention, and are intended to provide an overview for understanding theinvention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will become more apparent from a reading of the followingdescription in connection with the accompanying drawings in which:

FIG. 1 is an isometric view of a cylindrical lens array;

FIG. 2 is an isometric view of a prismatic lens array;

FIG. 3 is an isometric view of an array of hyperfine lenses having ahigh-density factor;

FIG. 4 is an isometric view of an array of hyperfine lenses having alow-density factor;

FIG. 5 is an isometric view of a blazed-type hyperfine grating;

FIG. 6 is an isometric view of a sinusoidal-profile hyperfine grating;

FIG. 7 is a front view of a fixture of the mold assembly in which themolding/embossing method provided by the invention may be carried out;and

FIGS. 8A, B, and C are sectional views of pairs of mold blocks, whichmay be used in the apparatus shown in FIG. 7.

FIG. 9 depicts a cross-sectional schematic of surface-relief microlens.

FIG. 10 is a graph plotting sag of a microlens as a function of Δn fordifferent numerical apertures and a focal length of 1000 micron meters(μm), see Eq. (3). Note that the sag of the microlens decreases as theindex of refraction difference increases.

FIG. 11 depicts a schematic of a blazed grating.

FIG. 12 shows the profilometer trace of the mechanical surface of amicrolens-type profile molded from a chalcogenide glass. The master wasa titanium alloy substrate machined with a mill to create a 513 micronmeters (μm) deep divot.

FIG. 13 shows profilometry data for a chalcogenide-molded diffractivelens having a 9 μm zone spacing. The fidelity of the replicated zonesare excellent and the measured width of the zone transition isessentially limited by the lateral resolution of the imaging apparatus.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides, in part, a one-step process offabricating complex optical components, such as gratings, lenses ofshort focal length, or high-density microlens arrays (MLAs). The processovercomes the molding limitations of other glass molding methods.Optical lenses have had various uses in many diverse technologies orapplications, such as in television or computer displays (e.g., LCDs).

To be technically precise, molding refers to a process of shaping aductile or fluid starting material to a final object and the embossingrefers particularly to a process of imprinting a design on the finalobject. Molding and embossing, however, are synonymous for the purposeof this invention. As used herein, the term ‘molds’ and ‘masters’ arealso synonymous: the mold is normally used for shaping process and themaster is used for imprinting designs.

As used herein, the term “fine microstructure” refers to a single lens,a lenslet or an array of lenses having smooth, curved features less thanor equal to about 500 microns along at least one dimension. Theindividual lens elements may be concave or convex; spherical, asphericalor fresnel; cylindrical (as illustrated in FIG. 1) or prismatic (FIG.2); and disposed on either a planar substrate or one a curved substrate.Arrayed lenses may be produced in various densities. In a “high-densityfactor” array, as shown in FIG. 3, the lens elements abut or lie closeto one another; an a “low-density factor” array, as shown in FIG. 4, thelens elements are spaced wider apart. In all four illustrations, thedimension “a” is less than or equal to about 500 microns.

As used herein, the term “hyperfine microstructure” refers to a singlelens, a lenslet or an array of lenses or microlenses having smooth,curved features less than or equal to about 100 microns along at leastone dimension, preferably less than or equal to about 10 microns.

As used herein, the term “fine grating” refers to a blazed-type grating,as illustrated in FIG. 5, or surface-relief diffuser, with groovespacing “a” less than or equal to about 500 microns; or a curved-profilegrating, as illustrated in FIG. 6, with groove spacing “a” less than orequal to about 500 microns. The depth of the grooves can be up to about100 microns deep. The groove spacing may be variable or fixed. Thegrooves themselves may be disposed on a planar or surfaced surface.

As used herein, the term “hyperfine grating” refers to a blazed-typegrating, as illustrated in FIG. 5, or surface-relief diffuser, withgroove spacing “a” less than or equal to about 100 microns, preferablyless than or equal to about 10 microns.

Depending on the particular material, hyperfine gratings and lenses mayreflect or transmit optical radiation. Generally, the molding,isothermal pressing and/or embossing of oxide glasses arehigh-temperature processes, involving temperatures in the range of about700-1200° C. or greater. Oxide-based glasses, particularly glassescontaining silicates and lead oxides are highly reactive in theirsoftened state. To prevent chemical reaction between the mold materialand the softened glass, expensive mold materials, such as chemicalvapor-deposited (CVD) silicon carbide, reaction bonded silicon nitridesand hard refractory metals and alloys, are required along with releasecoatings on the molding surfaces.

As an alternative, researchers have turned to sol-gel processes toproduce highly transparent glass, relatively opaque ceramics, orhyperfine-featured surfaces, such as described in U.S. Pat. No.5,076,980, or PCT Application Publication No. 93/21120, which areincorporated herein by reference. Workers in the field believed that tomold hyperfine features in glasses would be rather difficult sincevitreous materials retain significant viscosity at practical workingtemperatures, which would prevent the molten glass from accurately andreliably conforming to the hyperfine-featured mold.

We have found, however, that non-oxide glasses, in particularchalcogenide glasses, function very well for molding hyperfine featuredsurfaces in contrast to common optical or non-optical glasses thatcontain some type of oxide, such as oxides of silicon, aluminum, boron,lead and the like. For many specialized applications in optics,opto-electronics, and optical telecommunications, more particularly,development and application of non-conventional glasses may be the onlypractical material for engineers to use. It is believed that the abilityto produce consistently by a molding process optical elements, such asmicrolenses or diffractive gratings, having sharp transition angles withfeatures on the order of microns or submicrons, is not obvious fromprevious glass molding technologies using either oxide or non-oxideglasses.

Chalcogenide glasses are distinguished in their material compositionsfrom conventional optical glass families, in that they contain in theirglass-forming matrix a chalcogen element instead of oxygen. A chalcogenelement may be one or more elements of the sulfur group (e.g., S, Se, orTe) in the periodic table, and may be combined with arsenic, antimony,germanium, phosphorous, gallium, indium, etc. Additionally, chalcogenelements may be mixed with a halide (fluorine, chlorine, bromine,iodine) to create chalco-halide glasses. Since sulfides generally reactwith ambient oxygen at high temperatures, it was thought that thereactivity of such glasses would be a major obstacle for development ofa molding process. The chalcogenide glasses, however, were unexpectedlyresilient to chemical reaction in ambient atmosphere, and has littlepotential for undesired deformation or contamination during themanufacturing process.

Moreover, chalcogenide glasses have very interesting properties, whichfurther distinguish them from conventional oxide-based glasses.Chalcogenides exhibit excellent optical transparency in the near and farinfrared (IR) spectral region (>700 nano-meters (nm)). This is animportant attribute of chalcogenide glasses for fabricating opticallenses, since optical telecommunication uses transmittance in theinfrared spectrum. Silicates, by comparison, tend to absorb or areopaque in the mid-IR. Moreover, chalcogenides may be used in heatsensing applications, such as for forward looking infrared (FLIR)systems, or guidance in the nose of a missile. Certain chalcogenideglasses have potential applications as infrared transmitting materialsand as switching devices in computer memories, because theirconductivity changes abruptly when particular threshold values areexceeded. Moreover, chalcogenide glasses can function as semiconductors,not insulators as are most common oxide glasses, and are better thermalconductors; thereby, having the potential for better thermal managementwhen packaged in modules for telecommunication uses.

Other advantages of chalcogenide glasses include high refractiveindices. One does not necessarily need to have an aspheric lens whenlenses can be made from a non-oxide glass with a higher index ofrefraction. Generally, chalcogenide glasses exhibit higher refractiveindices in the range of about 1.8 to greater than 3, preferably ≧2.0,which affords much flexibility in design parameters, such as sag of alens or period of a grating. Using smaller sag, one can produce lenseswith reduced radii of curvature, and optical refractive lenses with lessdistortion in the optical pathway than oxide glasses with lowerrefractive indices of ≦1.5. Thus, a spherical lens is not a handicap inthis situation. The distance of the vertex to the plane of substrate isless. Hence, the sag required is smaller and shallower. Chalcogenideglasses also exhibit third-order, non-linear refractive index of about80 to 1000 times higher than that for silica; and, their phonon levelenergy is very low (˜300 cm⁻¹), which makes them an excellent host foroptical amplifiers or lasers doped with rare earth element ions (e.g.,erbium (Er), neodymium (Nd), praseodymium (Pr), thulium (Tm), ytterbium(Yb), etc.).

In contrast to oxide-based glasses, chalcogenide glasses exhibit lowersoftening temperatures and low glass transition temperatures, Tg(˜10^(13.4) poises). This feature makes chalcogenide glasses attractivecandidates for molding or embossing. As used herein, the term “low-Tg”refers to a glass that has a Tg≦about 500° C. Oxide-based glasses suchas silicates typically have a high Tg over about 600° C. and are proneto chemically react with the material of the mold. Although phosphateglasses have T_(g)˜300-320° C., and can be molded at about 400-450° C.,they have refractive indices that are considerably lower thanchalcogenide glasses. Hence, phosphates require greater sag to producecomparable lens or other optical elements.

Chalcogenide glasses can be molded at temperatures of about 200-600° C.or less, typically about 250-350° C., depending on composition, sincethey characteristically have glass transition temperatures (T_(g)) ofless than about 500° C. Although some chalcogenide glasses have Tg inabout 350-480° C., more commonly, the Tg is less than or equal to about300° C. (e.g., ˜130° C. or ˜190° C. to ˜200° C. or ˜250° C.). Hence, onecan take advantage of the low temperature properties of chalcogenides toachieve adequate fluidity to permit ‘hyperfine structure’ molding.

Chalcogenide glasses generally have coefficients of thermal expansion,on the order of about 10-50 ppm/° C., or for the examples in Table 1, ofabout 20-40 ppm/° C. The thermal coefficients of the mold materialselected may be in the order of about 2 to 40 ppm/° C., preferably about5-30 ppm/° C. Considering the field of Ge—As-sulphide glasses, thecompositional range of moldable glasses, which would satisfy theparameters of the mold, includes about 0-35% Ge, about 0-55% As, about30-85% S. To modify the optical, thermal, and/or mechanical propertiesof these glasses, phosphorus (P), gallium (Ga), indium (In), selenium(Se), tin (Sn), antimony (Sb), tellurium (Te), thallium (Tl) chlorine(Cl), bromine (Br), and iodine (I) may be added as optical constituents.Other elements, such as the rare earths, or fluxes (e.g., Li, Na, K) maybe also included.

In Table 1, we compare the material and optical properties of a fewselect chalcogenide and oxide-based glasses. For two chalcogenideglasses, Examples 1 and 2, Table 1 gives their properties and respectiveconcentrations in atomic percent of germanium, arsenide, and sulfur.Example 3 is a low-Tg (˜330° C.) oxide-based glass having afluorine-free composition, as disclosed in U.S. Pat. No. 5,021,366,incorporated herein by reference. The glass of Example 3 is currentlybeing manufactured and distributed to companies that mold opticalelements (e.g., Geltech, Orlando, Fla.; Eastman Kodak, Rochester, N.Y.).Example 4 is an oxide-based glass by Schott Glass Technologies, Inc.,and Example 5 is fused silica (HPFS®) by Corning, Inc. One notes fromTable 1 that the Tg of each of the chalcogenide glasses is lower thanthat of the other oxide-based glasses, including that of Example 3.TABLE 1 Material & Optical Properties of Select Chalcogenide &Oxide-based Glasses Ex. 4 Ex. 1 Ex. 2 (Oxide glass, Ex. 5 (8.75% Ge,(12.5% Ge, Ex. 3 Schott Glass Fused Silica 17.5% As, 25% As, (Phosphateglass, Technologies, (HPFS from Property 73.75% S) 62.5% S) Corning,Inc.) Inc.) Corning, Inc.) Index of ˜2.37 ˜2.48 1.60 1.51 1.46refraction (n_(D)) at the sodium D line, 589 nm Index of 2.186 2.284refraction (n₁₅₄₉) at 1549 nm dn/dT (ppm/° K) NA NA −11.0 2.5 10 (inNitrogen) Wavelength 530 −>2400 560 −>2400 400-1600 350-2000 200-2000transmission range (nm) CTE (ppm/° C.) 40 20 15 7.1 0.52 Density (g/cm³)2.72 3.03 3.80 2.51 2.20 Tg (° C.) ˜150-160 ˜245 330 559 1585 Ts (° C.)˜261 ˜348 Softening Point

In general, chalcogenide glasses will be more likely than conventionaloxide-based glasses to oxidize or otherwise react chemically in air whenheated. Despite this, however, we have discovered surprisingly thatchalcogenide glasses (Ge—As sulfides) may be molded in air withoutundergoing any detectable oxidization. Example glasses of Ge₂₅As₅₀S₆₀and Ge_(8.76)As_(17.5)S_(73.75)—respectively have Tg of 245° C. and 150°C, and CTE of 20 and 35 ppm/° C. Thus, when employing chalcogenidesglasses according to the present invention, the need to use an inertatmosphere during the processing of the glass is either abated oreliminated, which is a great advantage.

A number of other of suitable chalcogenide glasses will include, forexample, sulfide glasses that have a composition comprising, inatomic/element percent, about 25-90% S, 0-50% As, 0-45% Ge; selenideglasses comprising, in atomic/element percent, about 25-100% Se, 0-60%As, 0-45% Ge; or, telluride glasses comprising, in atomic/elementpercent, about 25-90% Te, 0-50% As, 0-45% Ge. A specific type ofgermanium-arsenic-sulfide is described in U.S. Pat. No. 6,277,775,incorporated herein, which contains a source of phosphorus ion as aco-dopant to effect dispersion of a rare earth metal ion dopant in theglass. Co-assigned U.S. patent application Ser. No. 09/894,587,incorporated herein, describes another kind of chalcogenide glass, whichcontains molecular clusters. Another specific chalcogenide exampleincludes a Ge—As selenide glass having a composition in atomic/elementpercent of about: 12.5% Ge, 25% As, 62.5% Se, with a Tg of about 219° C.

Alternative kinds of glasses may include chalco-halide glasses.Chalco-halide glasses are similar in composition to the samplechalcogenides except for the addition of Cl, Br, and I. A typical systemwould be glasses encompassed by the member components As—S—I, where Tgcan range from below room temperature for very I-rich species to about250° C. for I-poor compositions. Similar glasses exist in the systems:As—S, Se—Cl, Br; Ge—S, Se—Cl, Br, I and Ge—As—S, Se—Cl, Br, I, as givenin the review paper by J. S. Sanghera et al., J. Non-Cryst. Solids, 103(1988), 155-178, incorporated herein by reference.

Another major class of chalco-halide glasses are the so-called TeX orTeXAs glasses, containing Te and a halogen X with or without acrosslinking element such as As. For thermally stable lenses, the TeXAsglasses would be more preferred over the TeX glasses. Typical examplesof these and other chalco-halides are presented by J. Lucas and X-H.Zhang, J. Non-Cryst. Solids 125 (1990), 1-16, and H-L. Ma et al., J.Solid State Chem. 96, 181-191 (1992), incorporated herein by reference.

Halide glasses also may be employed for applications according to thepresent invention. Particular glass examples may be drawn from the widefamily of fluorozirconate glasses of which a typical example, referredto as ZBLAN, has a composition in terms of mole percent of about: 53%ZrF₄, 20% BaF₂, 4% LaF₃, 3% AlF₃, 20% NaF, with a Tg of about 257-262°C. Other possible halide glasses include the Cd halides of which thefollowing is a typical example: 17% CdF₂, 33% CdCl₂, 13% BaF₂, 34% NaF,and 3% KF, with a Tg of about 125° C. Broad compositional ranges forthese kinds of halide glasses are given in U.S. Pat. No. 5,346,865,incorporated herein, which include: 42-55% CdF2 and/or CdCl2, 30-40% NaFand/or NaCl, 2-20% total of BaF2 and/or BaCl2+KF and/or KCl, withoptional halides as listed.

These two illustrative halide glass families are not necessarily fullyinclusive of all halides as there are also fluorindate and fluorogallateglasses in which the major constituents are typically alkaline earthfluorides, (e.g., ZnF₂, CdF₂ and InF₃ and/or GaF₃). Having Tgs similarto that of ZBLAN, Tgs for these glasses can range from about 260-300° C.These glasses. A representative example is: 19% SrF₂, 16% BaF₂, 25%ZnF₂, 5% CdF₂, 35% InF₃, with a Tg of 285° C. When molding halideglasses according to the present invention, it is preferred that anon-reactive coating be used with the mold material to prevent thehalide species from reacting with air.

The method of the present invention, in part, is adapted from aproprietary process developed by J. Mareshal and R. Maschmeyer atCorning Inc., which is described in U.S. Pat. Nos. 4,481,023, 4,734,118,4,854,958, and 4,969,944, the contents of which are incorporated hereinby reference. According to the Mareshal-Maschmeyer process, a glasspreform having an overall geometry closely approximating that of thedesired final product is placed into a mold, the mold and preform arebrought to a temperature at which the glass exhibits a viscosity between10⁸-10¹² poises, a load is applied to shape the glass into conformitywith the mold, and the thereafter the glass shape is removed from themold at a temperature above the transformational range of the glass andannealed.

In contrast, the present inventive molding process does not require thatone use either molten or solidified glass gobs, or that preforms be in anear final shape. The glass material to be molded or embossed may be inthe form of regularly or irregularly-shaped bulk-solids, such as ingotsor a disc or wafer. For example, one can place a glass wafer, from 0.25to 2 mm in thickness and 50 to 300 mm in diameter, in between two halvesof the mold. Alternatively, fine glass frit powders (e.g., less than 0.1mm in diameter particle size) may be used. When glass frit is used, thepowder contains particles of sufficiently small, irregular-sized glassparticles to enable them to consolidate in the heated mold when pressureis applied. The powder consolidates initially to form a preform (such asa wafer, a gob or a rough-shaped lens or grating), which surprisinglycontained little if any occlusions. This ability to use glass materialsof virtually any shape can reduce fabrication costs and simplify themolding process.

A wide variety of temperatures and molding pressures may be employedsuccessfully to form glass articles of high precision, provided thatcertain minimum criteria are met:

First, the molding operation will be conducted at temperatures at whichthe glass has a much higher viscosity when compared with customary glasspressing procedures. Thus, the glass will be molded at viscosities ofabout 10⁶-10¹² poises, with a preferred range being about 10⁷-5×10¹¹poises, and a more preferred range of about 10⁸-10¹⁰ poises. Anynon-oxide glass composition may be deemed a suitable candidate for theinventive molding process, provided a suitable mold material isavailable, which is capable of being fashioned into a good surfacefinish, is sufficiently refractory to withstand the pressing temperatureand pressure, and is not substantially attacked by the glass compositionat molding temperatures.

Second, the inventive molding operation will involve an ostensiblyisothermal condition during the period wherein the final figure of theshaped article is being formed. As employed herein, the term isothermalmeans that the temperature of the mold and that of the glass preform, atleast in the vicinity of the mold, are approximately identical. Thetemperature differences permitted are dependent upon the overall sizeand specific design of the final glass shape, but the difference will,preferably, be less than 20° C. and, most desirably, less than 10° C.This isothermal condition will be maintained for a period of sufficientlength to allow the pressure on the molds to force the glass to flowinto conformity with the surface of the mold.

Normally, the glass products molded in accordance with the inventiveprocess contain too much thermal stress to be suitable for use inoptical applications and, therefore, a fine annealing step is demandedafter molding. Because of the isothermal environment utilized in thepressing procedure, however, and the fact that the molded articlesessentially totally conform to the mold surfaces, the articles shrinkisotropically, thereby permitting them to be fine annealed without anysignificant distortion of the relative surface figure. Moreover, thisannealing without distortion can be achieved outside of the mold with noelaborate physical support for the molded shape. This practice leads tomuch shorter mold cycle times and precludes the need for recycling themolds. In summary, there is no need to cool the mold under load with theglass shape retained therewithin to a temperature below thetransformation range or transition temperature of the glass. That is,the molds can be held at temperatures where the glass is at a viscosityof about 10¹³ poises (the minimum temperature at which the pressedarticles are removed from the molds), rather than cooling the moldsbelow the transformation range, perhaps even to room temperature, andthen reheating. Such cycling consumes much energy and adversely affectsthe life of the mold.

An important detail in the development of a glass molding or embossingprocess is the choice of material from which the mold or master isformed. The mold material should be chemically stable at the operationalmolding temperature, and it should not react chemically with the glassduring molding. In other words, the material of choice for the mold,which defines the profile or pattern to be formed on glass, preferablyshould possesses a similar thermal expansion to that of the glass andhave a recrystalization temperature that is substantially higher thanthe temperature to which the mold is heated. Usually, it is preferredthat the mold has a coefficient of thermal expansion (CTE) that issubstantially compatible with the CTE of the glass material. A materialwith a CTE that satisfies this and the other criteria, however, has beendifficult to find. Design parameters need to take account of the largecontraction of the glass that occurs when the mold cools when there is alarge disparity between CTE of the mold and that of the glass. (Itshould be noted that, at times, selecting mold/master materials andglasses of differing CTEs may be necessary to take advantage of thedifferential in expansion and/or contraction to help with the release ofthe molded/embossed parts from the molds/masters.) Although matching theCTE of the mold material and the glass is important, more important isthat the mold material should have a recrystallization temperature ofthat is substantially higher than the operational temperature of themold.

Although silicon carbide (SiC), silicon nitride (Si₃N₄), refractoryceramic composite of the two could be used to form the mold, ceramicmaterials, however, can be expensive and require coatings. Refractorymetals (such as tungsten) and alloys may also be used as molds, but somematerials such as titanium alloys are preferred in practicing thepresent invention Normally, a large difference in the coefficient ofthermal expansion exists between a metal alloy surface compared to thatof a glass. A titanium alloy, such as Ti-6Al-4V alloy, however, wasunexpectedly discovered to be very suitable for embossing and molding ofchalcogenide (sulfide) glasses, particularly glasses with a coefficientof thermal expansions in the range of about 20 to 40 ppm/° C. Thetitanium alloy has a coefficient of thermal expansion of within therange of approximately 8 to 12 ppm/° C., and a recrystallizationtemperature in the range of about 700 to 800° C.

Molds made from titanium alloys can be used to process chalcogenideglass blanks up to a temperature of about 500-550° C., which should besuitable for molding such glasses having a Tg of up to about 450° C. or500° C. The titanium alloy mold is made preferably of a nominalcomposition, in terms of weight percent, of about: 80-98% Ti (titanium);10-1% Al (aluminum); 10-1% V (vanadium), preferably consistingessentially, in weight percent, of about 90% Ti, 6% Al, 4% V.Commercially available Ti-6Al-4V alloys are normally used in structuralapplications for chemical industries and also in military aircrafts, butnot for manufacturing glass molds. According to the present invention,the mold blocks can be made of ceramics and their composites, metals oralloys, preferably of titanium or nickel alloys, particularly Ti-6Al-4Valloy, where the glass to be processed into optical components is anon-oxide glass, more particularly a chalcogenide type of glass.

The specific structure of the molding apparatus is not critical to theoperation of the inventive process. The press should contain somemechanism for moving the molds against the glass preform and someconstraints against the motion of the molds. Such constraints aredemanded to achieve the geometrical relationships required among theoptical surfaces. It will be appreciated that such constraints may beconstructed in a variety of ways. An apparatus developed in thelaboratory for molding lenses are illustrated in FIG. 7, which isexemplary only and not limiting. Hence, for example, the addition ofmechanisms for automatic loading and unloading of the glass, alternativesources of heating, cooling and press motion, and assignment of theessential functions to separate or different mechanical elements areconsidered to be within the technical competence or ingenuity of aworker of ordinary skill in the art.

FIG. 7 shows a face-on view of a fixture of a mold assembly 8 used. Theassembly 8 accommodates a base plate 10 on which a stationary mold halfis disposed, standing on insulator standoffs 14. A moveable mold half 16is guided by guideposts 18. A mechanism (not shown) connected to aguidance device 20 for the mold assembly on the mold half 16 isconnected to an actuator for applying pressure via the mold halvesagainst glass material to be molded or embossed. This material issupported between two mold blocks 22 and 24. The blocks are captured incavities in the mold halves 12 and 16. The mold halves are heated byelectrical heater elements 26 located in each mold half 12 and 16. Themold halves may be split to capture the heater elements 26.

FIGS. 8A, B, and C, depict a sectional view of three alternate pairs ofmold blocks used in the apparatus of FIG. 7. The mold masters of FIG. 8Bmay have various forms, such as spherical and aspheric, singleplano-convex, plano-concave, array of such lenses, and lens arrays. Themold blocks may be cylindrical in shape. The opposite mold surfaces 28and 30 of the mold blocks 22 and 24 define the profile of the opticalcomponent or element to be molded. Profiled cavities on the moldassembly will form various types of lenses on one side of a substrate ofthe glass material being molded. The profile of the molded object may beof either a convex or concave spherical lenses defined by a singleconcave semi-spherical mold cavity 32 (cavity for concave lens profileis not shown) in the mold surface, as shown in FIG. 8A. Alternatively,the mold cavity described can be of aspheric shaped (not shown) toproduce aspheric glass lenses. For a lens array, such as depicted inFIG. 8B, of convex (FIGS. 3 or 4) or concave (not shown) lenses theremay be multiple cavities 32 of semi-spherical or aspheric (not shown)shapes. Other profiles, such as double convex or double concave may beformed even on opposite sides of the glass substrate which is formed inthe mold, as for example with the mold blocks 22 and 24 shown in FIG.8C.

In the fabrication of surface-relief microlenses (41) on a substrate(42), one critical parameter is the sag of the lens, s. As defined inFIG. 9, this parameter designates the height that the vertex (45) of themicrolens extends above the substrate surface (43). For a microlenshaving a spherical shape of radius R, the sag is given by Eq. (1):s=R−{square root}{square root over (R ² −(D/2) ² )},   (1)where D is the diameter of the microlens. The radius of curvature R iscalculated from the indices of refraction of the incident medium n₁ andthat of the substrate n₂ and the desired focal length f according to Eq.(2):R=(n ₂ −n ₁)f   (2)Substituting Eq. (2) into Eq. (1), one obtainss=fΔn[ 1−{square root}{square root over (1−(NA/Δn) ² )}],   (3)where Δn=n₂−n₁ and NA is the numerical aperture of the lens defined by1/(2F/#), where the f-number (F/#) of the lens is f/D. Graphing the sagof the microlens s as a function of Δn for a fixed f-1000 μm, butvarious numerical apertures one obtains the curves represented in FIG.10. One notes that as Δn increases, the sag of the microlens decreases.By reducing the sag of the lens, one increases the manufacturability ofthe lens (e.g., in a molding process, it is less likely that air will betrapped in the mold if the sag of the microlens is less). One of themost important reasons for using high-index glasses, however, is forreducing optical aberrations.

Spherical surfaces are not ideal surfaces from an imaging perspective.To image a given object, one ideally requires an aspheric surface inorder to reduce aberrations. Aspheric surfaces, however, are morecomplicated to fabricate and to test than spherical surfaces. For lownumerical apertures (<0. 1) the error between the desire asphericsurface and the manufacturable spherical surfaces is generally small.For larger numerical apertures, however, this deviation increases,thereby degrading the image quality of the optical system. By usingmaterials of higher indices of refraction (e.g., chalcogenide glasseswith n₂>2.0), one can reduce the spherical aberrations of lens since therequired radii of curvatures for a set focal length is reduced.

For a blazed grating, a substrate (51) is patterned with a series ofblazed facets (52) across one of its surfaces, see FIG. 11. The desireddepths of these facets is given by${d = \frac{\lambda}{\Delta\quad n}},$where λ is the wavelength of operation and Δn=n₂−n₁. The depth of thegrating therefore decreases linearly with Δn. By decreasing the depth ofthe grating, one decreases the width of the shadow region (53) of thegrating. The shadow region is the region in which from a geometricaloptical analysis there is no light due to the shadowing caused by thesidewalls of the grating facets. The shadowing reduces the efficiency ofthe grating in the order of interest by introducing unwanted diffractioneffects. By reducing the depth of the grating (e.g., by increasing Δn),one increases the theoretical diffraction efficiency attained by thegrating.

According to an embodiment for carrying out the present molding method,the glass material, either in the ingot, wafer or powder form, is placedbetween the opposing mold surfaces 28 and 30 of the mold blocks 22 and24. Both upper and lower halves of the mold blocks or masters 22 and 24are heated simultaneously. Heating and cooling rates and dwell times aredetermined based on working conditions and particular glass species, andmay be controlled precisely using a digital temperature controller. Ingeneral, by increasing power to heating elements 26, the mold or masteris heated at a predetermined rate to an operational temperature that isat least about 10° C. above the Tg of the specific glass material in themold. Typically, the operational temperature is within a range of about10-110° C. above Tg. In other words, the glass has a viscosity of about10⁶ to about 10¹² poises. Preferably, the temperature is about 20° C. toabout 90° C. above Tg. More preferably, the temperature is about 30-70°C. above Tg. For certain chalcogenide glass samples that have a Tg inthe range of approximately 100° C.-350° C., or about 130° C.-250° C.,the operational temperature of the blocks 22 and 24, preferably areabout 50° C. above Tg, or in the range of 150° C. to 400° C.

Concurrently with heating of the precursor glass material, apredetermined pressure is applied mechanically to the upper half of themold against the lower half to form the optical components into thedesired shapes and with hyperfine microstructures (i.e., lens ormicrolens array or diffraction grating, diffractive optical pattern orcombinations of such lenses and patterns). A mechanical driver, forexample a screw drive, connected to the guidance devices 20 on the uppermold half 16 may be employed to actuate the pressing. Alternatively, themold assembly 8 can be put between the platens of a hydraulic press orelectrically driven press, such as a machine to apply pressure on themold blocks 22 and 24. As with heating, the exact amount of pressuredepends on various factors, including the Tg of the glasses or thecomplexity of the features in the profile to be molded. The pressure(force) of the mold translates to about 10 to 5000 pounds per squareinch (psi) of the mold surface area. For example, for glass speciesmentioned as suitable for molding according to the present process, thepressure can be in the range from 1 to 100 psi. Press the mold when theviscosity of the glass reaches about 10⁷-10¹¹ poises, preferably about10¹⁰ poises. After reaching the pre-selected peak operationaltemperature, the pressure is held on the mold block assembly for a dwelltime, suitably from about 0.1 to 10 minutes to ensure the completion offlow of glass material within the mold profiles or the designedpatterns. For a certain composition of a specific chalcogenide glass, adwell time, 10 sec., at the optimum temperature, 50° C. above Tg, andpressure, 20 psi, was utilized for completion of movement and filling ofthe viscous glass within the mold cavity. Then the pressure wasgradually released. The mold is allowed to cool at a predetermined rate,20° C./min., to 50 C or poises, and the optical component having thedesired profile is extracted from the mold assembly 8. For industrialfabrications, it is preferred that the temperature of the mold is notcooled all the way to room temperature and also use of cooling usingforced air or nitrogen. Additional process steps for removing the moldedproduct are either described by Mareschal and Maschmeyer or familiar tothose in the art.

For certain glass compositions, it may be desired to apply a releasecoating on the opposing mold surfaces 28 and 30 of the mold halves 22and 24. The release coatings may include: graphite carbon coating,molybdenum-di-silicide, fluorocarbon (CF_(x)), boron nitride, noblemetals and alloys, and some commercially available release coatings.Boron nitride (BN) was determined to be the best release coating formolding/embossing of sulfide glasses. The release coating material canbe effectively spray coated or sputter deposited on the mold surfaces.It will be noted that the molding and embossing processes in thisinvestigation are carried out in ambient air. An airtight enclosurehaving an inert atmosphere or vacuum was not required for such moldingand embossing processes. In the course of working with these materials,however, it was noticed that born nitride coated mold surfaces hadlonger service lives (i.e. more cycles of molding/embossing beforerepolishing the mold surfaces) compared to non-coated mold surfaces.

The foregoing general description of the method for fabricating opticalcomponents in glass and the apparatus for executing the method should betaken as illustrative and not limiting of possible variations ormodifications. The examples in the following section further illustrateand describe the advantages and qualities of the present invention.

EXAMPLES

In a series of studies, it was endeavored to mold and/or emboss opticalcomponents having fine, complex microstructures in compact shapes andsizes, using non-oxide glasses from either bulk-solid or powder forms (awafer, a cube, a irregular shaped agglomerate or powder) in the mold.Optical components, such as MLAs, required a variety of complex shapesfor densely packed individual lenses, aspheric lenses, or diffractiongratings, which may be used in optical switches, optical displays andthe like. In particular, chalcogenide glasses, in which oxide speciesare absent, impart unexpectedly favorable molding characteristics. Forthe experimental examples described below, all moldings were carried outin air. No inert atmospheres such as nitrogen or argon was used duringmolding. In some specific cases, forced air or nitrogen gas was used tofacilitate cooling of the molded parts and their removal.

The glass transition temperatures (Tg) of the chalcogenide glass sampleschosen for experiments ranged from about 160° C. to about 245° C. andindices of refraction were in the range of about 2.3 to about 2.5. Theprecursor glass material was in the form of a wafer having thickness inthe range of 0.25 to 2 mm and 20 to 300 mm in diameter. Alternatively,fine glass powders (less than 0.1 mm in diameter), and cube or irregularshaped solid block of glass were also used as precursor material. Notethis is in contrast with the preforms required in the molding processesdeveloped by Kodak (EP 1 069082 A2), Corning (U.S. Pat. No. 4,481,023),and possibly, by Geltech. We placed the glass, either in wafer or inpowder form, between the opposing mold surfaces of the mold blocks. Thetemperature of the mold blocks was increased from room temperature toabout 50° C. above the Tg of the specific glass material in the mold byincreasing the power input to the heater elements. The temperature ofthe blocks was in the range of 220 to 300° C. in the case of thechalcogenide glasses used (Table 1, Examples 1 & 2). Concurrently withheating of the glass precursor material, we applied mechanical pressurein the range of 1 to 100 psi to the upper mold portion. The amount ofpressure applied depended on factors such as the spatial frequency anddepths of the features in the profile to be molded as well as the Tg ofthe glasses. In order to apply pressure on the mold blocks, we placedthe mold assembly between the platens of a hydraulic or electricallydriven press, such as an Instron machine. After reaching thepre-selected peak temperature, the pressure was applied and was held onthe mold block assembly for a dwell time, typically from about 5 to 60seconds, to ensure the complete flow of glass material within the mold.After this dwell time, we gradually released pressure and reduced thetemperature of the mold block back to room temperature, as programmed inthe temperature controller.

After investigating several mold materials (including titanium and itsalloys, aluminum and its alloys, and steels, particularly series 440stainless steels), a specific titanium alloy, Ti-6Al-4V alloy, and/orelectroless high phosphorous nickel alloy were chosen as suitable moldmaterials, specifically for their stability at the molding temperatureused for molding these sulfide glasses. These materials havecoefficients of thermal expansion (CTE) of approximately 10 to 20ppm/°C. and a recrystallization temperature in the range of 700 to 900° C.Oxidation of some of the mold materials, particularly aluminum andiron-based alloys, prevented the use of those materials.

We molded precision microlenses, such as those required to collimatefibers, to measure the change in surface profile (e.g., radius ofcurvature) between that of the master and that of the molded glasses.The mold cavity was machined using a carbide tool and thendiamond-polished using of 10 μm grit. The higher precision masters wemolded were fabricated utilizing single point diamond turning. But themold material was not subject to any other special treatment, except toanneal for relief of stresses due to machining.

The present invention can replicate microstructures as deep as 500 μm inchalcogenide glasses in a fraction of an hour. Such a process can conveycommercial advantage in the manufacturing of cost-effective, precision,optical microstructures for optical surface-relief elements. Incontrast, reactive ion etching techniques can take as long as 12 to 24hours to etch 50 to 100 μm deep microlenses into fused silica. Thecapital expense of reactive ion etching and the associated supportequipment and human resources is significant and is typically measuredin millions of dollars.

FIG. 12 illustrates the result of a microlens-profile molded using theglass of designated Example 1, in Table 1 (8.75% Ge, 17.5% As, and73.75% S), as the fine precursor frit (˜10 μm). For this experiment, thetitanium mold or master had a concave surface-relief structure ofdiameter of 3 mm clear aperture and a sag of 513 μm. In order to moldthis type of structure, the mold surface was coated with BN by aerosolassisted spraying. After the frit powder was deposited in the moldcavity at room temperature, the mold was raised to 300° C. at a rate of20° C./min. The material was held at 300° C. for about 5 minutes beforea force of 20 psi was applied. The force was released after about aminute and the mold was cooled down to room temperature at a rate of 1°C./min. One notes from the profile trace in FIG. 12 that the moldingprocess had no difficulty replicating a 513 μm deep structure.

The molding of microstructures as fine as 5 μm can be resolved alsousing the glass of Example 1. At an operational molding temperature andpressure of only 245° C. and 50 psi, respectively, we were able tofabricate a 1.3-mm diameter, 0.94-μm deep, diffractive lens having 5 μmas its smallest grating period, using a single-point diamond-turned,high-phosphorous electroless nickel substrate as the mold master. Table2 summarizes the experimental conditions. For this experiment, the waferof Example1 was inserted into the mold only after it had already reached245° C. Based upon the sharpness of the diffractive zones achieved, itbelieved that the resolution of the Example 1 glass is significantlyless than 5 μm. Potentially, the resolution limit of the present moldingprocess and chalcogenide glasses can be refined and used to replicatewavelength dispersion gratings with grating periods of approximately 1μm or finer, such as for wavelength division multiplexer (WDM) modules.TABLE 2 Parameters for Molding Experiment Parameter Value Glass Example1 in wafer form (5 mm diameter, 2 mm thickness) Master High-phosphorouselectroless nickel Release coating None Max. temperature 245° C.Pressure 50 psi Molding time 30 min. (heating, pressing, cooling)

Afterwards, the molded-glass, diffractive, optical microstructures wereexamined using a Zygo NewView 100 for surface roughness and gratingfeature fidelity. The surface finish of the molded structure was foundto average 55 Å rms, while the master had a surface finish of 40 Å rms.FIG. 13 illustrates the 2-dimensional and 3-dimensional profiles ofdiffractive structures replicated according to the present invention.The fidelity with which the structures (grating grooves of ˜9 μm orlarger) were reproduced was excellent. To the lateral resolution of theNewView (1 μm), we observed no signs of degradation of the sharp zonetransitions when comparing those of the master to those of the replica.Gratings with a 2-D cross-section of about 8 μm or about 5 μm wide zoneperiodity can be made for diffractive lens. Certain precautions may beadvisable, however, such as applying release coatings to overcomeadhesion issues. Although adhesion issues can degraded the quality ofsub-8 μm period zones, the viscosity of the glass of Example 1 wassufficiently low enough to replicated with excellent fidelity the sharpzone transitions of the nickel diffractive master. Hence, it is believedthat the viscosity-limited resolution of the molding process usingExample 1 is significantly less than 5 μm.

In addition to investigating the resolution limits of the process, wealso investigated different release coatings. A number of releasecoatings, such as graphite carbon, molybdenum-di-silicide, fluorocarbon,boron nitride, some commercially available release coatings (e,g, ZincStearate Mold release, Thermoset release, Dry Film Mold release, RocketRelease etc. manufactured by Stoner Incorporated, Quarryville, Pa.) wereevaluated from the criteria of their effectiveness in improving thesurface smoothness of the molded optical components and the life cycleof the mold surfaces. For each release coating, we evaluated theireffectiveness in maintaining the surface quality of the mold cavity andthe lifetime of the release-coated mold surfaces. From a number ofexperiments, it was determined that boron nitride (BN) was the mosteffective release coating material of the group for the molding. Boronnitride coated mold surfaces, particularly for the titanium alloy molds,had longer service lives (i.e., more cycles of molding/embossing inbetween cleaning or repolishing of the mold surfaces) compared tonon-coated mold surfaces.

In general, the coatings for releasing surface-sensitive components fromthe molds are best applied either by physical or chemical vapordeposition techniques. In the absence of thin-film coating facilities,the release coating material may be effectively spray coated on the moldsurfaces. Spray coating on polished mold surfaces, however, tends toincrease the roughness of the mold surfaces because of the particulatenature of the spray-coating material. As a result, the molded lenssurfaces replicated the roughness of the mold surfaces caused byspray-coatings. Sputter-deposited films/coatings are more preferred foroptical surface release due to the smoothness of the resulting releasecoating surfaces. For chalcogenide, in particular sulfide glasses, andchalco-halide glasses, surprisingly, a release coating was not requiredwhen titanium alloy and high phosphorous electroless nickel molds wereused. Other nickel alloy surfaces also do not necessarily require arelease coating.

The present invention has been described generally and in detail by wayof examples and the figures in detail and by way of examples ofpreferred embodiments. Persons skilled in the art, however, canappreciate that the invention is not limited necessarily to theembodiments specifically disclosed, but that substitutions,modifications, and variations may be made to the present invention andits uses without departing from the scope of the invention. Therefore,changes should be construed as included herein unless they otherwisedepart from the scope of the invention as defined by the appended claimsand their equivalents.

1-30. (canceled)
 31. A precision optical element made according to amethod comprising the steps of: a) providing a non-oxide glass with aglass transition temperature (Tg) of up to about 550° C.: (b) providinga mold having an active surface that has an optical finish, wherein saidactive surface if made of a titanium alloy with a composition, in termsof weight percent, comprising about 98-80% Ti. 1-10% Al, and 1-10% V;(c) placing said glass in said mold, (d) heating said mold and glass toan operational temperature from about 10° C. to about 110° C. above theTg: and (e) pressing the mold when the viscosity of the glass reachesabout 10⁶-10¹² poise; wherein, optionally, said titanium active surfaceis coated with a protective coating, said coating being one selectedfrom the group consisting of: (A) a release agent: and (B) a materialhaving a crystallization temperature higher than at least an operationaltemperature, said material being further coated with a release agent.32.-55. (canceled)
 56. A precision optical element formed from anon-oxide glass by a molding or embossing method, wherein the methodcomprises: providing a glass having a glass transition temperature (Tg)up to 550° C. as granular, planar, or bulk-solid items; providing a twopart mold having an active surface with an optical finish, which may beused with or without a protective coating, wherein said active surfaceis either optionally: (A) coated with a layer of non-reactive material,or (B) made from either a titanium alloy or a nickel alloy, or (C) bothmade from either a titanium alloy or a nickel alloy and coated with saidnon-reactive material; charging said mold with said glass, heating saidmold to an operational temperature of at least 10° C. above said Tg; andhot-pressing said glass.
 57. The precision optical element according toclaim 56, wherein said temperature is at least about 50° C. above saidTg.
 58. The precision optical element according to claim 47, wherein themethod further comprises inserting blocks into said mold, at least oneof said blocks presents a section that faces said wafer or powder. 59.The precision optical element according to claim 47, the method furthercomprises placing on a surface of said blocks a layer of non-reactivematerial that is non-reactive with said glass at said operationaltemperature.
 60. The precision optical element according to claim 47,wherein said non-reactive material is boron nitride.
 61. The precisionoptical element according to claim 31, wherein said non-oxide glass is achalcogenide glass.
 62. The precision optical element according to claim31, wherein said non-oxide glass is a chalcogenide glass is selectedfrom the group consisting of arsenic sulfide, germanium sulfide andgermanium-arsenic-sulfide glasses.
 63. The precision optical elementaccording to claim 62, wherein, in atomic/element percent, germanium isin the range of 0-35%, arsenic is in the range of 0-55% and sulfur is inthe range of 30-85%.
 64. The precision optical element according toclaim 31, wherein said non-oxide glass is a chalcogenide glass isselected from the group consisting of arsenic selenide, germaniumselenide and germanium-arsenic-selenide glasses.
 65. The precisionoptical element according to claim 31, wherein, in atomic/elementpercent, germanium is in the range of 0-35%, arsenic is in the range of0-55% and sulfur is in the range of 30-85%.
 66. The precision opticalelement according to claim 31, wherein said non-oxide glass is achalcogenide glass is selected from the group consisting of arsenictelluride, germanium telluride and germanium-arsenic-telluride glasses.67. The precision optical element according to claim 31, wherein, inatomic/element percent, germanium is in the range of 0-45%, arsenic isin the range of 0-60% and selenium in the range of 25% to about 100%.68. The precision optical element according to claim 61, wherein tomodify the optical, thermal and/or mechanical properties of said opticalelement's chalcogenide glass, said glass further comprises one or moreelements selected from the group consisting of phosphorus, gallium,selenium, tin, antimony, thallium, chlorine, bromine, iodine, a rareearth element, lithium, sodium and potassium.
 69. The non-oxide glassaccording to claim 31, wherein said glass is a chalco-halide glass. 70.The non-oxide glass according to claim 31, wherein said glass is ahalide glass.
 71. The precision optical element according to claim 56,wherein said non-oxide glass is a chalcogenide glass.
 72. The precisionoptical element according to claim 56, wherein said non-oxide glass is achalcogenide glass is selected from the group consisting of arsenicsulfide, germanium sulfide and germanium-arsenic-sulfide glasses. 73.The precision optical element according to claim 72, wherein, inatomic/element percent, germanium is in the range of 0-35%, arsenic isin the range of 0-55% and sulfur is in the range of 30-85%.
 74. Theprecision optical element according to claim 56, wherein said non-oxideglass is a chalcogenide glass is selected from the group consisting ofarsenic selenide, germanium selenide and germanium-arsenic-selenideglasses.
 75. The precision optical element according to claim56,wherein, in atomic/element percent, germanium is in the range of0-35%, arsenic is in the range of 0-55% and sulfur is in the range of30-85%.
 76. The precision optical element according to claim 56, whereinsaid non-oxide glass is a chalcogenide glass is selected from the groupconsisting of arsenic telluride, germanium telluride andgermanium-arsenic-telluride glasses.
 77. The precision optical elementaccording to claim 56, wherein, in atomic/element percent, germanium isin the range of 0-45%, arsenic is in the range of 0-60% and selenium inthe range of 25% to about 100%.
 78. The precision optical elementaccording to claim 71, wherein to modify the optical, thermal and/ormechanical properties of said optical element's chalcogenide glass, saidglass further comprises one or more elements selected from the groupconsisting of phosphorus, gallium, selenium, tin, antimony, thallium,chlorine, bromine, iodine, a rare earth element, lithium, sodium andpotassium.